Developmental arrest within a host allows many parasitic nematodes to evade adverse external conditions and action of chemotherapeutic agents, but to reactivate at opportune periods. Our long range goal is to elucidate the molecular mechanisms that trigger reactivation of arrested parasites and to identify strategies of preventing neonatally- transmitted infections as well as self-reinfections (e.g. Strongyloides in immunodeficient individuals). This study is focused on pregnancy- induced reactivation and transmammary transmission of arrested Ancylostoma larvae because hookworms infect about 1.2 billion people, and in young children, the protein loss and anemia can contribute to physical and cognitive deficiencies. We have previously shown that larval reactivation cannot be attributed to a state of generalized immunosuppression as is associated with pregnancy; rather, hormonally- induced cytokine changes appear to be critical. Using an in vitro reactivation assay, we find that TGF-beta stimulates larval feeding at levels comparable to that of serum stimulation. Further, the stimulatory effect of serum can be significantly neutralized with antibodies to TGF-beta. Earlier studies have shown that exogenous estrogen and prolactin stimulate a resurgence of larvae in the milk of latently-infected, lactating dogs, and more recent work indicates that these hormones specifically upregulate the levels of TGF-beta2 during late pregnancy and lactation. In addition, TGF-beta. signaling is clearly crucial to reactivation of arrested larvae of the soil nematode, Caenorhabditis elegans. The objective of this proposal is to use the mouse/A. caninum model to test the hypothesis that host-derived TGF-beta particularly the beta2 isoform which is upregulated by estrogen and prolactin, triggers the reactivation and transmammary transmission of developmentally-arrested larvae during late pregnancy and lactation. We propose to test our hypothesis with 3 specific aims: 1. Characterize the A. caninum homologues of the TGF-beta receptor and ligand to evaluate binding of mammalian TGF-beta to the parasite receptor. 2. Use the in vivo model of infection to determine TGF-beta levels, and larval status and burden in skeletal muscle versus mammary tissue during different phases of pregnancy and lactation, and determine whether larval reactivation is stimulated by estrogen and prolactin. 3. Use in vitro co-cultures of tissue larvae with muscle versus mammary cells to distinguish the differential effects of estrogen, prolactin and staged pregnancy sera on larval reactivation, and determine if neutralizing antibodies to TGF-beta inhibit reactivation. These studies will provide important results for the development of therapeutic strategies to eradicate latent parasitic infections.